docs: def is_within_publication_dates(obj, timestamp=None):
          """
          Return True if the given timestamp (or ``now()`` by default) is
          witin any publication start/end date constraints.
          """
          if timestamp is None:
              timestamp = timezone.now()
          start_date_ok = not obj.publication_date \
              or obj.publication_date <= timestamp
          end_date_ok = not obj.publication_end_date \
              or obj.publication_end_date > timestamp
          return start_date_ok and end_date_ok